While you can keep typing that headline after 70 characters, there is a good reason not to put important stuff past 70 characters. But there is a REALLY REALLY good reason for the count. You've heard about the Google spiders right? Well they read headlines. But they only read the first 70 (approximate) characters. So anything after that is NOT going to help index and rank your post. It's not just important on ActiveRain, but a habit you should adopt for your WP blog too if you have one.
I also want to take the time to point out to you the benefit of an ActiveRain "Rainmaker" membership right now too. If your post is members only it's going to be searchable within the community by other members and will show up on the blog roll, but it will NOT be indexed and ranked on Google.
The headline, in my opinion, is the very most important part of your blog. It's HUGE in your blog being found. When I started adding the words "new listing, affordable, cheap listing, foreclosure, bank owned combined with my usual key term of the area I specialize in "Lake Sinclair" my clicks on those posts doubled and tripled from the time before I learned that. So a headline that would perhaps be a search term you think your target consumers might type is the best headline you can write for a post.
That 70 character count made me realize that I needed to put the important stuff that Google would use to rank. On my other blog that allows longer titles I've learned to write the important stuff at the beginning of the post. The first 70 characters of the headline SHOULD be things you think potential clients will be searching for on line to find you. And it's important to put your key words at the beginning of your headline. Anything beyond 70 characters and you can expect that those words won't be indexed and help your SEO.
How does an ActiveRain post help your SEO on your own site? Link backs. Organic linkbacks to relevant searches and posts and pages on your site. ActiveRain gets a lot of traffic so Google likes it. I still have posts I wrote here years ago with live links to IDX searches for certain neighborhoods and still get regular traffic to my site via those older posts every week.
Oh and while I am at it, let me invite you to join the Ask an Ambassador Group. It's a great place to get your blogging/ActiveRain questions answered and learn a LOT about blogging from members willing to help and give you tips, suggestions and answers.